The easiest way to access the Task Manager is via right-clicking on the Toolbar at the foot of your screen (and then, unsurprisingly, clicking on 'Start Task Manager').
I suggest that you should do that as soon as you've booted up your computer (as it can take ages to start when everything is already running slowly). Click on the 'Processes' tab and then on 'Show processes from all users'. (If you're asked for permission to proceed, grant it). Look for programs that are shown with a high figure in the 'CPU' column or in the 'Mem Usage' (or 'Working Set Memory') column.
If, as sometimes happens, you find a high figure next to 'svchost.exe.', that will only tell you that all of your processing power is being taken up by something which is being download (or, at least, something which your PC is attempting to download). The usual culprits are antivirus programs that are trying to update themselves but you'll need to look more carefully down the list to see what it actually is that's making the call on svchost.exe. You do that by looking for a process where the Memory figure (and/or the CPU figure) keeps changing every second or two. (NB: The figures you're seeking might actually be quite small. It's not their size that matters but the fact that they keep changing; that's what alerts you to the fact that they're making a call on svchost.exe).
Also try temporarily disabling all of Norton 360's components, to see if that makes a difference. Also try running your PC without an internet connection.
Also, of course, run a malware scan using the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: At the end of the installation process, you'll encounter a pre-ticked box next to an invitation to take a free trial of the Premium version of the program. Ensure that you deselect it before proceeding). If your PC is running so slowly that you can't download Malwarebytes Anti-Malware, download the installation file to another computer and then transfer it to your via a USB memory stick.